1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly to integrated circuit fabrication processes and structures.
2. Description of the Background Art
Stress liners have been employed to enhance the drive current of transistors in integrated circuits. FIG. 1 schematically shows an example integrated circuit structure 100 for a metal oxide semiconductor (MOS) transistor employing a stress liner 105. In FIG. 1, the structure 100 has a gate 101 formed on a gate oxide 103, which is formed on a substrate 102. Active regions 104 (i.e. source and drain) are formed in the substrate 102. The stress liner 105 comprises silicon nitride deposited under process conditions that result in either tensile or compressive strain, depending on the application. A silicon nitride film under tensile stress is referred to as “tensile nitride” and tends to generate tensile strain in the substrate under the gate, while a silicon nitride film under compressive stress is referred to as “compressive nitride” and tends to generate compressive strain in the substrate under the gate.
A problem with stress liners is that although they may be beneficial for one type of MOS transistor, it may be detrimental to the opposite type. This is particularly troublesome in the manufacture of transistors fabricated using complementary metal oxide semiconductor (CMOS) technology. A CMOS integrated circuit typically has a complementary n-channel MOS (NMOS) transistor and p-channel MOS (PMOS) transistor on the same substrate. Although a tensile nitride film may be beneficial to an NMOS transistor, the same film may degrade a PMOS transistor and vice versa. This is graphically illustrated by the plots shown in FIG. 2, which is discussed in H. S. Yang, et al., “Dual Stress Liner for High Performance sub-45 nm Gate Length SOI CMOS Manufacturing,” IEDM Tech. Dig., pp. 1075-1078, December 2004. As shown in FIG. 2, while a tensile nitride film increases the drive current of an NFET, it lowers that of a PFET. Likewise, a compressive nitride film increases the drive current of a PFET but lowers that of an NFET.